Oscillating beam deflecting devices have been used, in the prior art, in a variety of embodiments. Typical apparatus is disclosed in U.S. Pat. No. 3,886,309 and 3,876,829. Disclosed in the referenced patents are display or image creating systems which include an oscillating mirror for deflecting a modulated optical beam from a source of such beam to an image area or a display area. The apparatus disclosed in the referenced patents are typical in that the oscillating mirror oscillates, of course, in two directions; however, only one direction of mirror oscillation is actually employed for deflection or reflection of the modulated optical beam, and the oscillation of the mirror in the other direction is unused and is considered analogous to the dead time encountered in a CRT flying spot scanner which is commonly referred to as the retrace time. Those skilled in the art will readily appreciate that the ability to utilize the retrace or deadtime in such a device is a desirable feature.
One solution in an optical scanning environment which is directed at a problem which has some similarities to the problem to which this invention is directed, is disclosed in U.S. Pat. No. 3,848,087. Although that patent is concerned with a rotating beam deflecting device, such as a rotating polygonal mirror, rather than an oscillating beam deflecting device, the patentee overcomes variations in mirror facet size and orientation by controlling the motor driving the beam deflecting device. While this particular solution may fit with the needs of the patentee, in general, apparatus in which control is exercised over the travel or velocity of the beam deflecting device to insure superimposition of images in a display system is to be avoided. The reason that solutions of this sort are not favored is that it is generally much more difficult to accurately control the positioning and/or the velocity of a mechanical system such as that represented by the rotating polygonal mirror, than it is to control some purely electronic device.
One reason for the failure, in the prior art, to use the time during which the mirror is returning to an initial position for actually creating an image or a portion thereof, has been the difficulty in synchronizing the beam source with the mirror, so that the beam scanned in one direction overlies or is superimposed upon the beam scanned in the opposite direction. This criteria is required for otherwise the image, for example, in a display system will be blurred. Typically, images are formed, for example, in a dot wise format, each dot being defined by a pulse produced by an oscillator or clock. In order to use mirror travel in both directions for beam reflection, some way must be found of assuring that the angle the mirror makes with the optical source when the last "dot" is written, is reproduced after the mirror has reversed its travel and the beam is now being scanned in the opposite direction. Significant advantages accrue from double direction scanning, for example, brightness is increased as well as flicker frequency.
While the foregoing discussion is specific to an oscillating mirror (thereby implying an optical beam), those skilled in the art will appreciate that similar requirements are imposed on any beam deflecting system, i.e., the deflecting device is not necessarily a mirror.
It is therefore an object of the invention to provide, in an oscillating beam deflecting system, synchronizing apparatus to assure that a beam scanned in one direction of motion of the beam deflecting device will overlie or be superimposed over the beam scanned in the other direction of motion of the beam deflecting device. It is another object of the invention to provide synchronizing apparatus for an oscillating mirror in a display device to assure that an image created during mirror travel in one direction overlies or is superimposed on an image created by the travel in the opposite direction. It is another object of the present invention to provide such a device which does not rely for operability upon the ability to control mirror motion, but which rather has the ability to conform to variations in mirror motion.